Security documents, such as, in particular banknotes, securities, identity cards, identification cards, payment cards, etc. are supplied with a variety of security features to protect them against counterfeiting. Counterfeiting of security documents can be minimized by applying a variety of security features to a document of value so that expenditures associated with counterfeiting are as high as possible, making possible forgery operation as difficult and costly as possible, and optimally—unprofitable. Because forged documents are often executed using another (usually cheaper) technology and/or other materials than the originals, an effective safety element should be impossible to reproduce accurately using a different technology and/or using other materials than originally used. The security element should also be durable to use and recognizable for machines and equipment. At the same time its production/introduction into the structure of a security document should be as inexpensive as possible and should provide the possibility to easily integrate with existing production processes.
There are three levels of security:                a) the first degree—security based on organoleptic properties—for users who do not have any additional tools or equipment;        b) the second degree—securities verified using basic tools, e.g. magnifying glass, UV lamp, etc.;        c) the third degree—security verified by experts (specialists) in appropriately equipped laboratories.        
The oldest security elements used to date are all kinds of watermarks, easy to implement during the production of paper and practically impossible to produce on finished paper. Another technical element is opaque materials used for the execution of a document. Sometimes these additions can be used as markers in addition to the security functions. Covering materials with different additions, whose composition is the secret of the producer and the user, are the most common security. They are often placed on the border of a security available (recognizable) for an average document user, and security elements recognised by a special device only.
The used security features include:                special, secret recipe paper conferring specific mechanical and optical properties;        replacing paper with polymer substrate difficult to print on;        the use of microprinting;        recto-verso, i.e. printing on both sides, in which a picture visible in the lumen is created with the finely fitting elements located on both sides of a sheet, e.g. a banknote.        a description on the recto;        metallic foil hot-stamping (hot stamping) with a holographic model;        embossment resulting from intaglio printing;        complex graphics, yet distinct, with strong saturation and gloss paint;        watermark, especially visible against the light;        a security thread in the form of a metal strip inserted within the paper structure with spacing forming an inscription;        printing with an optically variable ink—seen in front and at a sharp angle it changes colour;        ribbing;        drawings visible under UV light.        
As modern analytical methods allow you to easily examine the structure of each thin-film product, effective protection should be based more on process. At present optimal security solutions for value documents against counterfeiting include a combination of traditional methods, IT technology and materials technology that uses nanomaterials.
Although the features of security elements to be verified either by a user or a machine are characteristics associated with the mechanical, electrical and thermal conductivity properties, etc., the features of the widest range of applications are optical properties of the security element.
The contents of U.S. Pat. No. 3,910,701 present a method and a tester for measuring the refractive index, absorption and/or light transmission, provided with a light emitting diode (LED) emitting light of different wavelengths and directing them to the test element, and at least one element sensitised, which is reached by light of each of the LEDs reflected from the surface and/or penetrating through the test piece. The described tester further comprises control means to the separate LEDs, which allows, among others, determination of the relative reflectance of various elements of the test at a particular wavelength, the differential reflectance for one test element at two different wavelengths and the permeation and absorption parameter corresponding to these measurements and the relative differential.
On the other hand, U.S. Pat. No. 5,894,352 discloses a method and a tester for determining the level of absorption of light in the light transmitting optical elements. The method is based on the measurement of the temperature rise of the optical element resulting from absorption of light.
Furthermore, DE 10126722 A1 discloses a hand-held device for testing the authenticity and/or validity of security document such as admission tickets or travel tickets. The device comprises a reader to move a security document through it and read information and/or security features contained in this document and a testing component, which verifies the authenticity and/or validity of a security document based on data recorded by the reader.
In parallel with the development of increasingly complex and difficult to counterfeit security features against counterfeiting, there is a continuing need for possible simple to use, compact, versatile, and independent testers and methods for detecting such high-tech security elements in security documents. As a rule, the operation of verifying the authenticity of a security feature in the tested document cannot lead to damage of the structure of the document.
One modern type of document security against counterfeiting are nanomaterials introduced directly into the structure of a security document (paper, plastic), or contained in a separate security element arranged on or within the structure of the document.
The nanomaterials with properties that are particularly useful in the field of document security against counterfeiting include graphene, which is a flat structure made of carbon atoms connected in hexagons. Graphene shows, among others, linear dependence of dispersion, resulting in a unique light absorption. Light absorption occurs when an electron from the valence band can absorb a photon (the photoelectric effect). This is possible if the energy difference between the point of the valence band and conduction is the same as the energy of the photon. In graphene in the area called “point K” there is a linear dispersion dependence and closed energy slot, which means that each wavelength of light (each colour) in the range of from near infrared to ultraviolet light can be absorbed by graphene, as there will always be an electron, which can absorb a photon. Furthermore, the probability of absorbing each wavelength of light is the same. Since the graphene is a single layer material, it absorbs only a very small part of the incident light (2.3%) and thus is a material having a high degree of translucency (transparency), and at the same time a material having a very strong absorption of light (as for such a thin structure).
The graphene security feature can be completely transparent or in the case of the need to obtain visualisation, it can have a form which is visible to the human eye.
Examples of graphene structures in security features include:                (a) a layer of graphene (two-dimensional or structured, e.g. nanotubes) arranged between two polymer layers;        (b) a layer of doped graphene (two-dimensional or structured, e.g. nanotubes) arranged between two polymer layers.        
The structure of graphene in a security element may be supplemented by the so-called quantum dots. Quantum dots (QD) are semiconductor nanocrystals with sizes ranging from 2-10 nm. Just as semiconductors, quantum dots absorb photons of light with such energy that gives you the ability to transfer electrons from the non-activated to one of the higher available energy levels. Otherwise, there is an emission process, because the wavelength emitted by light depends on the size of the dots. Hence, having a semi-conductive material we can get markers having different colours, which are characteristic of quantum dots.
The radiation may be absorbed by quantum dots in a broad spectral range, whereas their molar absorption coefficient increases towards the UV. Thus the excitation can be made of many kinds of dots using one light source, since there is no requirement to apply excitation at a pre-set wavelength. In turn, the profile of the fluorescence emission of quantum dots is narrow and has a small half-width value (FWHM 125 nm). This allows for simultaneous use of multiple markers having different colours without fear of overlapping of the signals. The nanocrystals can be repeatedly excited, with no noticeable decrease in fluorescence because they have a high quantum yield of fluorescence and long radiation (10-100 ns).
The structure of the nanocomposite material comprising a graphene security element takes into account:                the use of a variable number of layers of polymer in the material—the number of coats applied is dependent on the conditions in which the security element will operate.        The use of a variable number and size of the graphene quantum dots, if the use such structure gives is necessary to increase the efficiency of the security feature.        
Graphene security elements, as well as other such security elements have a uniquely shaped design, preferably forming a specified image. This shape is applied in the production process on the transparent polymer substrate. The process of applying the layer of a nanocomposite is compatible with the application methods for the corresponding polymers. The thus obtained security elements are then placed in known manner in the structure of security documents.
The objective of the invention is to provide a tester and detector of a graphene security element in a security document, such as a banknote, an identification card, etc., allowing for a simple, rapid and reliable verification of the authenticity of this document. This objective has been achieved by applying the solution set out in the appended claims.